1. Field of the Invention
The present invention relates to a sealing mechanism that is suitably applied to a rotor shaft system of the steam turbine and the like; thereby, the natural frequencies regarding the sealing mechanism can be easily determined; and, the vibration resonance between the mechanism and the rotor shaft system can be easily prevented. The present invention also relates to a steam turbine provided with the sealing mechanism.
2. Background of the Invention
A steam turbine is provided with an outer turbine casing, and an inner turbine casing inside the outer casing; in the center part of the inner casing, a rotor is rotation freely installed as apart of the rotor shaft. A plurality of rotor blades (a cascade of rotor blades) is fixed to the predetermined positions (locations of the cascades) along the longitudinal direction of the rotor shaft; thereby, the positions are arranged so that the distance between a position and the adjacent position is set in advance. On the other hand, a plurality of stator blades (a cascade of stator blades) is fixed to a stator blade circular-cylinder fixed to the inner casing so that a cascade of stator blades is placed between a cascade of rotor blades and the adjacent cascade of rotor blades. Thus, a cascade rotor blades and a cascade of stator blades are placed in turn in a multi-stage arrangement, so that the cascades of rotor blades and the cascades of stator blades form a steam flow passage; a steam inlet port is provided at the inner casing as well as the outer casing; the steam entering the steam inlet port streams through the steam flow passage so as to rotate the rotor and drive a generator coupled with the rotor shaft.
The rotor is rotation-freely supported by a bearing housing that is placed outside of the outer turbine casing regarding the steam turbine; the bearing housing is placed outside of the outer casing and fixed on a foundation made of concrete or the like. A part of the rotor shaft part between the bearing housing and the outer casing is covered with a gland (shaft seal part) provided so as to prevent steam from leaking outside; the gland is supported by the bearing housing via a connecting member; in usual practices, a part of low pressure steam that has been used for rotating the rotor and is discharged from the inside of the inner casing is guided to the gland.
Further, in order not to relieve the steam outside from the space around the rotor between the outer turbine casing and the gland, at least one bellows joint of a cylindrical corrugated shape is provided between the outer casing and the gland, so as to surround the rotor shaft; thereby, the bellows joint is able to absorb the relative displacement between the outer casing and the gland, the relative displacement being caused by thermal expansion or working pressure. Incidentally, to make sure, it is noted that the corrugation shape can be seen in a cross-section passing through the axis of the cylindrical shape, but not in a cross-section perpendicular to the axis.
JP 2009-235971 (herein referred to as “Patent Reference”) discloses a sealing mechanism configured with the bellows-joint arranged between the outer turbine casing and the gland. The sealing mechanism disclosed by Patent Reference is hereby explained with reference to FIGS. 4 and 5 attached in this specification. In the sealing mechanism 100A as shown in FIG. 4, a bellows-joint 102 (comprising an inner bellows-joint and an outer bellows-joint) of a double layer structure form a cylindrical corrugated shape so as to surround a space around a rotor shaft (not shown in FIGS. 4 and 5). The bellows-joint 102 are provided so as to connect a ring-shaped end flange 112 to a ring-shaped end flange 114; thereby, the ring-shaped end flange 112 is connected to an outer turbine casing (not shown in FIGS. 4 and 5), while the ring-shaped end flange 114 is connected to a gland (not shown in FIGS. 4 and 5); the bellows joint 102 gas-tightly seals the space around the rotor shaft between the outer casing and the gland prevent steam from leaking outside.
For instance, a steam turbine installed in a nuclear power plant is provided the double layer type bellows-joint (comprising an inner bellows-joint and an outer bellows-joint), from a safety point of view.
The bellows-joint 102 comprises an inner bellows-joint 102a and an outer bellows-joint 102b, both the bellows-joints 102a and 102b having a flat (cylindrical) part 104. The inner bellows-joint and the outer bellows-joint are formed or manufactured from two cylindrical tubes; the space inside the two superposed tubes is pressurized from the inside toward the outside so that the outer tube and the inner tube are together pressed onto a forging template. On the outer periphery surface of the flat part regarding the outer bellows-joint, a rib 106 of a ring shape is fastened; on the outer periphery surface of the rib 106, a plurality of holes is provided so that a plurality of weights 108 is inserted therein in order that the rib 106 and the weights 108 configure a vibration control means. Arranging the number of the weights 108 as well as arranging the mass of each weight enables the natural frequencies of the bellows-joint 102 to be adjusted; thus, the resonance between the rotor shaft system vibration and the bellows vibration can be evaded.
Patent Reference discloses another configuration example regarding the bellows-joint as shown in FIG. 5; whereby, a sealing mechanism 100B is provided with a bellows-joint 102 comprising an inner bellows-joint 102a and an outer bellows-joint 102b, both the bellows 102a and 102b not having a flat (cylindrical) part, and a bellows-joint 116 being arranged outside of the bellows-joint 102. In addition, both the left and right ends of the bellows-joint 116 are connected an end flange 112 or 114 via a supporting member 118. In this disclosure (Patent Reference), providing the bellows-joint 116 enables the natural frequencies of the sealing mechanism 100B to be adjusted.